1. Field of the Invention
The present invention relates to an image formation apparatus and, more particularly, to an image formation apparatus which develops an electrostatic latent image formed on a recording medium by a developing unit to which an AC bias voltage is applied.
2. Description of the Prior Art
In a conventional image formation apparatus for performing electrostatic recording such as an electrophotographic copying machine, various image formation conditions are controlled in accordance with the conditions of an original such as the texture.
For example in order to control the image formation conditions by discriminating the density of the original, the density of the leading end of the original is sensed and the exposure is controlled in accordance with the obtained result. Alternatively, the developing bias is varied to adjust the gradation so as to form an optimal image. Still alternatively, the original is scanned while the exposure is corrected in real time, thus forming an optimal image.
In order to develop an electrostatic latent image formed on a recording body such as a photosensitive drum according to the magnetic-brush developing method, the characteristic curve of the development density as a function of the drum potential has a relatively steep leading edge as indicated by a curve (A) (solid line) shown in FIG. 4, since the distance between the photosensitive drum and the developing sleeve is great. This results in development with poor gradation. In order to form an optimal image, the leading edge characteristics must be changed.
However, according to the conventional control method of the image formation conditions as described above, the characteristic curve of the development density as a function of the drum potential during development cannot be controlled. For example, characteristic switching may not be effected between an image including mostly continuous tone portions such as a photograph and an image of black and white such as characters. Gradation may not be well controlled.